The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking (hereinafter FCC) of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium. The FCC process is carried out by contacting the starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. During the cracking reaction, coke will be deposited on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interfaces with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, if it is desired to increase the conversion rate an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Since the catalyst temperature in the regeneration zone is usually held at a relatively constant temperature, significantly higher than the reaction zone temperature, any increase in catalyst flux from the relatively hot regeneration zone to the reaction zone affects an increase in the reaction zone temperature.
The hydrocarbon product of the FCC reaction is recovered in vapor form and transferred to product recovery facilities. These facilities normally comprise a main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked products which usually include bottom materials, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams.
As the development of FCC units has advanced, temperatures within the reaction zone were gradually raised. It is now commonplace to employ temperatures of about 525.degree. C. (975.degree. F.). At higher temperatures, there is generally a loss of gasoline components as these materials crack to lighter components by both catalytic and strictly thermal mechanisms. At 525.degree. C., it is typical to have 1% of the potential gasoline components thermally cracked into lighter hydrocarbon gases. As temperatures increase, to say 1025.degree. F. (550.degree. C.), most feedstocks can lose up to 6% or more of the gasoline components to thermal cracking.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor and are transported upward by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums if present upon contact with the hot catalyst. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds. A number of riser reaction zones use a lift gas as a further means of providing a uniform catalyst flow. Lift gas is used to accelerate catalyst in a first section of the riser before introduction of the feed and thereby reduces the turbulence which can vary the contact time between the catalyst and hydrocarbons.
In most reactor arrangements, catalysts and conversion products still enter a large chamber for the purpose of initially disengaging catalyst and hydrocarbons. The large open volume of the disengaging vessel exposes the hydrocarbon vapors to turbulence and backmixing that continues catalyst contact for varied amounts of time and keeps the hydrocarbon vapors at elevated temperatures for a variable and extended amount of time. Thus, thermal cracking can again be a problem in the disengaging vessel. A final separation of the hydrocarbon vapors from the catalyst is performed by cyclone separators that use centripedal acceleration to disengage the heavier catalyst particles from the lighter vapors which are removed from the reaction zone.
In order to minimize thermal cracking in the disengaging vessel, a variety of systems for directly connecting the outlet of the riser reactor to the inlet of a cyclone are suggested in the prior art. Directly connecting the cyclone inlet to the riser outlet in what has been termed a "direct coupled cyclone system" requires a means for relieving pressure surges that can otherwise overload the cyclones and cause catalyst to be carried over into the product stream separation facilities located downstream of the reactor. The development of these systems to handle the overload problem in a variety of ways increases the practicality of directly coupling the riser outlet to the cyclone inlet. Direct coupling of cyclones can greatly reduce thermal cracking of hydrocarbons.
It is also known, for purposes of controlling thermal cracking, to lower the temperature of the reaction products upon leaving the cyclone separators by the use of a quench liquid. Quenching the product stream reduces its temperature below that at which thermal cracking can occur and reduces the loss of gasoline products by continued cracking to light ends.